Building wall



R. R. KELLER BUILDING WALL Dec. 15, 19 4 3 Sheets-Sheet 1 Filed NOV. 18,1959 Dec. 15, 1964 R. R. KELLER 3, 6

BUILDING WALL Filed Nov. 18, 1959 3 Sheets-Sheet 2 R. R. KELLER Dec. 15,1964 BUILDING WALL 3 Sheets-Sheet 3 Filed NOV. 18, 1959 United StatesPatent Ofifice 3,151,2h7 Patented Dec. 15, 1964 3,161,267 BUILDING WALLRobert R. Keller, 150 Milford Sn, Manchester, Nli. Filed Nov. 18, 1959,Ser. No. 853,790 1 Claim. (Cl. l89--34-) This invention relates tobuilding walls and more particularly to wall panels having largecontinuous surface areas, for use as outer and inner walls of buildingsand for partitions. The application is a continuation-in-part of mycopending application, now issued as Patent No. 2,931,468.

The existence of thin, high-grade sheet materials, plastics, fibers,minerals and metals, has caused hope that bricks, lumber and concretewould be replaced by more economical building walls which are highlyimproved in appearance, much lighter and hence easier to erect and whichrequire little or no maintenance throughout a long life. But, inaddition to remaining less expensive, conventional materials have beenregarded as more reliable in appearance and in resistance to impactloading.

Lightweight panels made from available highgrade thin sheet materialshave generally had an unsightly, unevenly dished appearance. Such panelsplaced side by side have not presented a consistent appearance, butrather uneven shadow effects have occurred, due to the unevenly dishednature of surfaces of the panels. Such panels have also had low impactresistance being damaged in appearance by impinging impact loads andfrequently in structure as well. Stiff high-grade sheet materials (e.g.,high-density cement-asbestos) have had such low resistance to impactloading as to have been completely unusable for walls in ofiicebuildings and others, where long life and good appearance are importantfactors. Panels without at least one of these defects have not been ieconomically competitive with conventional materials, and have,therefore, found application only where expense has been of littleimportance.

The principal object of this invention is, therefore, to produceimproved building wall panels of high-grade, thin sheet materials.

Another object is to provide wall panels having a consistent, evenappearance. Panels following the invention can have glossy, smooth,highly polished outer skin surcan, undished and completely planar.

Another object is to provide panels having very thin surface skinmembers with high impact resistance.

A still further object is to provide lightweight wall panels constructedwith a minimum of expensive materials, having a large span betweenstiffener members.

Achievements of the invention include the provision of prefabricatedbuilding panels having good insulation and sound absorption features andwhich provide fire barriers.

The invention comprises an interior, low-density sheet member having amutual extent with and bonded continuously to the inner surface of acontinuous high-grade skin member, but having spaced-apart elongatedinterruptions, and spaced-apart, elongated rigid stiffener membersbonded directly to the skin member at the interruptions. Where thepanels comprise structural members, and not mere curtain walls, a secondskin is bonded on the other side of the stiiftfener members.

The low-density thick backing sheet material must have the followingcharacteristics: It must be relatively thick, a thickness of and upwardsto 1", preferably between /2" and A". It must have a non-friable facedirected toward the high-grade skin member, non-friable being used inthe sense that surface material must not rub off or otherwise separatefrom the general mass of lowdensity material. This non-friable surfacemust conform to the matching surface of the skin member. The lowdensitysheet must have substantial structural stability and stiffness. Wherethe sheet is fibrous, affected by moisture to become less stiff, uponthe face opposite from the bonded skin there must be a vapor barrier,preferably a very thin impermeable sheet such as polyethylene oraluminum coated paper, laminated thereto.

Skin members according to the invention must be continuous weather andwear resistant sheets of a thickness generally less than /8", in manypreferred embodiments less than .080" and must be high-grade: highlycompact with hard and non-friable surfaces and having a relatively highinternal strength with a tensile yielding point generally above 1000p.s.i. where thicknesses around 4;" are employed, and proportionatelyhigher specific strengths for thinner members. The high-grade materialmay be fairly brittle as is high-density cement-asbestos, but notextremely brittle as in glass.

A continuous film of flexible adhesive must bond together the matchingsurfaces of the interrupted lowdensity sheet and the skin memberthroughout their mutual extent.

The rigid, elongated stiffener members fitting the interruptions in thelow-density sheet have non-friable conforming surfaces directed towardsthe inner surface of the continuous skin member. A film of high-gradeadhesive material is interposed, directly bonding each stiffener memberto the skin. In structural panels a second continuous skin member, alsoof high-grade material is generally parallel to the first skin member,separated therefrom by the stiifeners and is bonded through mtachingnon-friable surfaces to them. Where both sides of a structural panel areexposed to impact loads and to view, a low-density sheet member isbonded to each of its skin members.

The invention is further amplified in the following detailed descriptionincluding illustration of a number of preferred embodiments in thedrawings wherein:

FIG. 1 is a perspective, exploded view of a typical prefabricatedstructural wall panel constructed according to the teachings of thisinvention;

FIG. 2 is a section of the panel of FIG. 1 along line 2-2;

FIG. 3 is a perspective partially cut-away view of another preferredembodiment of a prefabricated structural wall panel;

FIG. 4 is a section of the panel of FIG. 3 taken on lines 44;

FIG. 5 is another section of the panel of FIG. 3 along line 5-5 thereofshowing in particular the construction of the core;

FIG. 6 is a section similar to that of FIG. 5 of another preferredembodiment of the invention;

FIG. 7 is a section similar to FIG. 4 of another preferred structuralpanel of the invention for use where both surfaces are exposed to viewand to impact loads;

FIG. 8 is a fragmentary perspective of a large prefabricated curtainwall panel in which plastic stiffeners are employed; and

FIG. 9 is a magnified, partially cut-away perspective of an embodimentwherein a moisture barrier is employed.

Thickness of the skins and the films of adhesive and clearances betweenlow-density sheet interruptions and stiffener members are generallyexaggerated for the purposes of illustration.

As noted above there exists a number of high-grade materials produced invery thin continuous sheets.

Skin walls of many of these high-grade materials yield slightly atvarying rates across the surface thereof in response to heating,cooling, wind conditions and other factors, dishing in irregular amountsacross the surface, forming a very undesirable dished appearance. In thecase of planar panelsthis dishing is characterized as a non-planarirregular surface with high points appearing at points of attachment ofthe skins and irregular depth .low points across the surface between.Increase in thickvness of the skin decreases'the dishing tendency-butsuch improvement is made only by greatly increasing the cost .per unit.areaof the wall, and theweight thereof. The invention eliminatesdishing in a panel surf-ace so that instead of increasing the thicknessof the expensive high- .grade sheetmater-ial, it is possible to utilizemuch thinner skin memberspermitting substantial savings.

The invention, in addition, permits great-increases in theimpactresistance-of skins. This impact. resistance concerns thewithstanding of such concentrated loads as may occur when stones orbaseballs are thrown against .a building wall; when air rifles aredischarged thereagainst; andfrom impacts from kicking or 'scufling ormoving furniture thereagainst. To be acceptable, walls mustwithstandsuch impact loads, not only without structural failure, but withoutsurface changes such as crack- :ing, ,denting, pitting and crazing.

Following the invention relatively brittle materials,

heretofore unsuitable because of cracking and failure under low impactloads (e.g. high-density cement-asbestos), can now be utilized inpermanent structures, .e.g., as outer walls of oflice buildings; dentingby local stretching beyond the elastic limit as in extremely thin metalsheets isavoided; and crazing as occurs in plastic sheets is eliminated.7

All of this is accomplished while providing a structure in which centerdistancesbetween elongated stiffener membersare unusually great,providing a minimization of .relatively heavy and expensive stiffenermaterial.

- 4 the interior of this second skin is bonded low-density bagasse board15 interrupted and bonded as is sheet 14. The structural panel thicknessmay range up to 8", thickness between 1 /2 and 4 being presently mostcommon.

The preferred embodiment of FIGS. 3, 4 and 5 is a prefabricatedstructural panel having a height of 20 feet and a width of 4feet. Thepanel 40 comprises an open stiffener frame 42 comprised of I sectioncore elements 43, 43" and 45 of rigid structural material providingoppositely directed bonding areas bonded directly to outerhigh-gradeskins 44, 46 with high-grade bonding material 48. Skin 44,.isa high-grade continuous wear resistant sheet (stainless steel of athickness of about .020) extending across the entire width'and length ofthe panel; skin 46 is also a high-grade continuous sheet (high-densitycement-asbestos of -a thickness of about /sn). I

An interrupted, interior low-density thick and stiff backing sheetmaterial 50 (low-density asbestos board) with a thickness of between /2andv has a non- .friable surface geometrically conforming with theinterior surface of skin 44. The'interruptions of thislowdensity sheet50 receive the stiffener members, but the low-density sheet 50 is notbonded to them. A layer of flexible bonding material 52, continuousand-coterminous with the low-density sheet portions is interposedbetween the matching surface of the low-density sheet 50 and theinterior surface of skin. 44. To the interior of the skin 46, bondedthereto in a like manner, is an interrupted low-density fiberboard 51 of/2" thickness, formed from reclaimed newspapers. V

The stiffener structure of this embodiment also follows my copendingapplication, as shown in FIG. 5. It will been seen that a longitudinalgroove 60 is provided along the abutted side of an, abutted element 43.A projection means at the end of .the abutting element 45,. hereoppositely directed cars. 49, is twisted into this groove preventingendwise' removal of element 45 away fi'om Q element 43.

polyester plastic, but may obviously be formed of other 7 such plastics,e.-g. vinyls, Bakelites (phenolic) and acrylics reinforced wherenecessary with paper, wire, glass, and the like.

Upon the interior area .of this skin is a low-density thick and stiffbacking sheet material 14 .of fiberboard formed from reclaimednewspapers, with a thickness of about /2" having a non-friable facetowards the skin and having spaced-apart elongated interruptions 16, 18.The low-density sheet covers the entire inner area of the skin exceptfor the interruptions, and is bonded thereto with acontinuous film 20 ofaflexible bonding agent, e.g., a plastic or rubber base adhesive.

Elongated rigid stiffener members 22, .24 fitting the interruptions areinserted therein. These stiffener members provide non-friable bondingareas conforming with the inner surface .of the skin member, and ahigh-strength bonding material 26 is interposed between the stiffenermembers and the matching skin area, effecting a highstrength direct bondtherebetween. The rigid stiffener members 22, 24 are strong andlightweight. In this embodiment the stiffener members 24 are box shapedwith I shaped members 22 having spaced-apart longitudinal slides 23 inwhich end projections 25 of abutting, members 22 interlock for slidingengagement following the teachings of my abovementioned copendingapplication, and flanges of members 24 and 22 oppositely directed fromskin member 12 are bonded to a second wear resistant skin member 30generally parallel to the first. This second skin, a high-grade sheet,is of higlrdensity melamine sheet .060 thick and is separated from thefirst by the stiffeners interposed therehetween. To

I In FIG. 6 are flanged'stiifener members ofanothe'r preferredembodiment which, while not offering the rigidity of structure of thoseof FIG. 5, are suitable in instances where the skin members offersubstantial strength. An abut-ted element 61 is abutted by abuttingelement 62. A longitudinal seating portion 64 (the inside surfiace of aflange) is provided along the abutted side of the abutted element 60. Aprojection means 66 is provided at anend portion of the abutting elementfor slidably engaging the seating portion, while maintaining theouter'flange surfaces of abutted and abutting elements in common planes.

During assembly, with slidable connections as shown in FIGS. 1 through6, as taught in my copending application, the abutting members may beselectively positioned along the sides of abutted'members so to fitwithin the elongated interruptions in the low-density sheet.

Other advantageous embodiments of my present invention do notem'ploy mynovel connection between stiffenermernbers.

Referring to FIG. 7 this prefabricated structural panel has a narrowwidth and is useful for curved walls. Both of the oppositely directedskin Wall surfaces have extremely high impact resistance and resistanceto dishing, and the panel is, therefore, useful to define both inner andouter walls of a gymnasium. Parallel opposed stiffener members '70 and72 have opposed U crosssections with oppositely directed flange bondingareas lying in parallel spaced-apart planes. High-grade continuous wearresistant skin members 74 and 76, each high-density cement-asbestos witha vitrified outer surface, thickness Vs", extend across the panel, onein each plane 'being bonded intimately to the flanges with high-'strength adhesives 78. On the" inside surface of one "ness, and to theother sheet member 82 is a low-density asbestos board thick, each ofwhich extends continuously across the area of its skin member withelongated interruptions only at the periphery for the stiffeners 70 and72. A film of flexible bonding material 73 is interposed between themutual extent of each skin member and its low-density sheet member.

Referring to FIG. 8 a curtain wall comprised of a network of rigid,rectangular cross-sectioned plastic stiffener members 84 and 86 notchedfor egg crate assembly are bonded to a high-grade weather resistant skin90 of aluminum of .015 thickness. To the entire inside area of sheet S0,interrupted only at the stiffeners, is bonded low-density thick andstiff backing sheet material hl, pressed paper sheet of a thickness ofReferring to FIG. 9 a sheet of low-density sheet material conforming tothe requirements of the invention is shown. This sheet has a thicknessof /z". The under surface A is non-friable and conforms to a skin memberE to which it is bonded with a flexible bonding agent D. The thick andstiff low-density sheet has a mutual extent with the skin member exceptfor elongated interruptions F. This sheet of low-density material isformed of compressed fibers and although stiff is subject to becomingflexible when subjected to moisture. Laminated to surface B, facingopposite from non-friable surface A is a vapor barrier layer C comprisedof a thin plastic sheet laminated thereto. Vapor barrier material isalso applied to the edges of the low-density sheet. Rigid Wooden andplastic stiffeners with an ordinary rectangular cross-section may beused to advantage in some instances to provide oppositely directed orsingle bonding areas.

To withstand varying wind loading, construction han dling and headloads, it is imperative that the rigid stiifeners have non-friablesurfaces so as to withstand differential expansion and contraction andmust be compatibly bonded to the high-grade skin sheet, at intervalsdirectly thereto. This high-grade bond must be stronger than thepermissible flexible bond existing between the interior low-densitysheet and the skin sheet as it must transfer stress and aiford support,and stiffness currently achieved only by the use of extremely expensivebonding agents. An example of such an adhesive is synthetic rubber basephenolic material. The comparatively narrow nature of the stiifenerbonding areas permits the use of such high-grade bonding agents incompetitive structures.

The following is a table of some advantageous combinations of high-gradeskins and low-density sheet material, combined, according to theteachings of the invention, with the stiffener members, to comprisepanels economically competitive with conventional materials. The thickand stiif low density backing sheet in every instance is upwards of A2"in thickness, preferably within the range of /2" to The face of thislow-density sheet bonded to the skin is non-friable. It has substantialstructural stability and stiffness, not highly pliable, and is providedwith a vapor barrier where subject to softening Water stiifener membersG are bonded directly to the skin memvapors.

EXAMPLES OF COMBINATIONS FOR BUILDING PANELS High-Grade SkinsLow-Density Sheet Added Advantages Stainless Steel 0.040" thickAsllgiztos fliggard is thick, weight Fire Barrier.

oz. Fiber Glass reinforced Polyester Bagasse (Pressed Sugar Cane Fiber)Extremely Light.

sheet .030 thick, weight 9 oz./it. High-density Cement Asbestos isthick, weight 20 oZ./ft.

Pressed Exploded Fiberboard 0.125

thick, weight 12% 01../it. Plywood 3-ply laminations,

thick over-all.

Anodized Aluminum 0.015 thick.

Enameled Steel 0.015 thick over-alL.

High-density Cement Asbestos Pt thick, weight 20 oz.,/ft

Aluminum 0.015 thick Fire Barrier, High Insulation Value.

High Sound Absorbency, Ideal Interior Partition.

High Insulation Value.

Very lightweight.

Inexpensive Permanence.

Inexpensive Medium Life.

Feather weight.

ber, conforming to the interruptions F of the low-density sheet.

As indicated in the drawings, FIGS. 1-7, some preferred forms of theinvention are structural panels, which are selfasupporting over largeareas and which can withstand wind loadings and head loads withoutadditional support. For these structural panels to resist buckling undersuch loads it is important that the sandwich be relatively deep, i.e.that the opposite skin sheets be spaced apart as high as 8". The depthprovides substantial stiffener material spaced from theneutral axis ofthe panel. The panel acts as a sturdy column, and is, therefore, notsubject to buckling failure.

Other preferred embodiments, e.g., P16. 8, comprise a single continuousWeathering wear resistant skin mem- High-density plastic, aluminum andsteel are examples of non-friable surfaced material which may be soutilized.

N0 dishing occurs with the above structures. Each combination hasextremely high impact resistance. Of course, other combinations ofmaterials under the limitations set forth also achieve improved buildingpanels. Among these are light-transmitting compact plastic facings,backed with light-transmitting slightly expanded plastic foam sheets ina light-diffusing structural panel.

Although low-density sheet materials are pound-forpound much lessdesirable from a structural tension and compression standpoint than thehigh-grade skin materials, they permit decrease in skin thickness to thepoint that the total weight of the panels is not greatly increased whiledishing is prevented. Thus an unbacked skin panel of a givenconstruction and weight which dishes badly and irregularly can be ofabout equal weight with a panel according to the invention, of similarconstruction, which does not dish. The reason for this is that thethickness of the interior sheet material as defined provides asubstantial amount of rather stiff material spaced apart from anypossible bending axis of the interior low-density sheet, preventingbending thereof and thus supporting a thinner skin from dishing eitherin or out.

The low-density sheet has a much poorer surface facing, more subject todamage under impact loading, such as cracking, scratching and denting,as compared with the high-density skin material. But it greatly improvesimpact resistance of the high=g rade skin sheets furnishing in bondedcombination with the skin, characteristics lacking in the thin Thinhighgrade skin members (compact, hard, non-friable surfaced, strong, notextremely brittle) standing alone,ha've anumb'er of impact resistancedeficiencies found in differing degrees from material to material.ductile and subject to permanent denting and wrinkling; harder plasticsare subject to surface pitting and crazing when impact loads impingethereupon. High-grade skin members less susceptible to these, areinversely much more brittle and subject to cracking and breakage, oneexample or this sort of material beingrthe thin, highdensitycement-asbestos sheet. All high-grade thin skins will have their impactresistance Substantially increased by this invention. The improvement:in impact resistance is primarily due to the combined effects ofdistributiono'f impact loads throughout substantial skin area' andimpact absorbency.

(A) Impact Spreading in Skin 7 As described above, the low-density sheetmaterial must not be pliable but rather must have structural stabilityand stiffness, of course, a function of the thickness of the material aswell as its particular internal structure. With this degree ofstiffness, upon a concentrated impact blow the force tending to deformthe skin is transferred through the impact point on the skin member to amuch larger area of the interior low-density sheet. In turn,'thesedispersed forces upon the interior low-density sheet, tending to depressit, are restrained by the adjacent portions of the bonded skin member atpoints spaced apart from the point of impact, tending to draw thoseportions of the skin in the direction of impact. A substantial area ofthe skin is thus slightly deflected and does the work of resisting aconcentrated impact blow with no portion of the skin being subjected tostresses in excess of the elastic limit of the material.

(B) Impact i lbsorlieitcy Impact resistance,'especially for those skinswhich are relatively brittle, lies also in the absorptioncharacteristics of the backing sheet. Cracking and breaking of brittleskins is due in large part to responsive vibrations in the brittlesheet. With the low-density sheet of the invention, the impact shock istransferred into the low-density sheet and is d ampenethand cracking andshattering are prevented.

Elasticity of the low-density sheet inherent in its stiffness, isimparted to the high-density sheet, achieving v greater impactabsorption without yielding.

(C) Decrease in Prestress In many previous structures skin members havebeen prestressed across stiffener members, in order to attempt toprevent dishing. When an impact load occurred, sufficient deflection ofthe skin to absorb the blow greatly increased the tension stressed atthe point of impact, exceeding the elastic limit and causing damage.With the instant'invention, however, prestressing to eliminate dishingis not needed and damage is avoided. v r

It must be appreciated that not one of these efiects alone explains thefull advantages of the invention in any one embodiment, but there is acooperation of eflects difficult to analyze. 7

To achieve the surface bond between the inner skin area and the matchinglow-density sheet adequate to provide the resistance to impact loadingit is imperative that each surface be continuous and non-friable, sothat a bond therebetween effects a lasting union between the mass of thelow-density sheet and the skinto withstand differential expansion andcontraction and to give impact resistance. The bonding material m'ust'beflexible so that the portions can flex in response to concentratedimpacts.

.ND numerical weight limits are employed herein in Metals and someplastics in thin sheets are are 1,12 6? 8 defining the low-density ofthe thick and stiff backing sheet member without reference to specificmaterial because avery loose composition formed of a material *havinghigh molecular weight would not have a non friable surface, stitfnes's'or structural stability, but would have a higher Weight per unit areathanfor instance, a satisfactory pressed fiber board of the'samethickness. What is generally intended by low=den'sity sheet which hasstructural stability and stiffness, is a'she'et of material having astrong rigid bridged network, but with substantial air space, and,therefore, being capable of being further compacted to a substantialamo'unt during its manufacture. Of course, to be useful herein, thesheet must have a non-friable surface.

As an example, presently known polyurethane expanded foarn having adensity upwards of 15 lb./ft. in sheets of thickness has a suflicientlystrong, rigid bridged network to serve -as the lowdensity sheet, whilesuch foams of lower density arefinadequate for the purpose having, forinstance, insufi icient :stability and stifiness for use in the instantinvention as well as an inadequate surface. I I

The specific stiffness of the low-density material (modulus ofelasticity and theelastic limit) is not specified with numerical limits'in defining low density of the sheet first, because the over-allstiffness of a sheet is a function of the thickness of the sheet whichmay be varied within the teachings of the invention over a substantialrange as well as the specific stiffness. Secondly, the required amountof over-all stiifness depends upon the particular application. As anexample, for a backing for skins of fiber glass reinforced plastics,dishing is the greatest problem to be combatted because the reinforcedplastic has a comparatively favorable impact resistance apart fromimpact crazing, due ,to the high elasticity of the fiber glass. Thus,although the backing material serves both important functions ofincreasing impact resistance and decreasing dishing, the nature of therequired stiffness of the low-density sheet is that the material berigid but need not have a very high elastic limit. Ctintraste'd withthis are skins of extremely thin sheets of steel, more ductile thanfiber glass reinforced plastic, being particularly subject to permanentimpact denting' and wrinkling by yielding, as well as dishing.

These thin sheets of steel require not only a stifi interior backingmember, but also one having elasticity over a widerange, providingelastic return and efficient spreading 'of stress to a wide area of theskin relative to the area of impact. High-density cement-asbestos sheetskin members, in further contrast, is weakest from an impact loadingstandpoint, and absorption of shock being very important, thelow-density sheet must have low compaction, and while stiff, is notrequired to have as high a degree of stiffness as is necessary in someof the other situations.

b The teachings herein serve as a complete guide to the assemblyrnan whowill find many combinations obvious, following the teachings herein,using other analogous materials than the examples herein within thespirit and teachings of the invention.

What is claimed is:

A prefabricated building'panel comprising a grid formed of amultiplicity of rigid grid members mechanically connected together, eachof said grid members having a web and flanges at each edge thereofextending at an angle to said web, the outer surfaces of each of saidflanges being substantially fiat and parallel to each other, the gridmembers extending in two directions and defining a multiplicity of openspaces surrounded by said grid members, said flanges at one edge of.said webs defining a first set of bonding surfaces, saidbonding'surfaces being aligned in a single plane, a first outer sheetmember extending in said plane over said grid, said first *sheet memberhaving an outer wearresistant surface and bonded with a layer ofadhesive face-to-face to said first set of bonding surfaces, said layerof adhesive lying directly between the cooperating bonding surfaces ofsaid first sheet member and said first set of bonding surfaces, saidfirst sheet member extending continuously over all of said open spaces,a multiplicity of stitf, pre-forrned backing sheet members one fitted ineach of said open spaces in said grid, each backing sheet member havingan area substantially corresponding to the area of said first sheetmember lying over said open space, each backing sheet member having aplanar bonding surface bonded by a layer of flexible adhesiveface-toface to said first sheet member substantially throughout saidarea, said backing sheet members each being thicker than said firstsheet member and structurally of lesser density than said first sheetmember and said grid members, the outer surfaces of said flanges on theother edge of said webs defining a second set of bonding surfaces, saidsecond set of bonding surfaces being aligned in a single plane, a secondouter sheet member adhesively secured to said 19 second set of bondingsurfaces, said second set of bonding surfaces locating said second sheetmember in a spaced apart relationship to said backing sheet members, thesmallest dimension of each of said open spaces parallel to said planesbeing substantially greater than the spacing between said planes.

References fitted in the file of this patent UNITED STATES PATENTS887,912 Bokor May 19, 1908 1,540,542 Carhart June 2, 1925 1,637,497ODowd Aug. 2, 1927 2,160,066 Frische May 30, 1939 2,302,586 Thelen Nov.17, 1942 2,757,116 Clements July 31, 1956 2,890,977 Bayer et al June 16,1959 FOREIGN PATENTS 723,621 Great Britain Feb. 9, 1955

